<p>Boundary quantum phases ---a special type of quantum phenomena--- occur in the boundary part of the system. The boundary part can be a surface of a bulk material, an interface between two distinct system, and even it can be a single impurity or a impurity cluster embedded into a bulk system. The properties of the boundary degree of freedom can be affected by many strong electron correlation effects, mesoscopic effects, and topological effects, which, therefore, induce a vast variety of exotic boundary quantum phases. Many techniques for precise fabrication and measurement in nanostructures had been developed,</p><p>which can provide ways to prob, understand, and control those boundary quantum phases.</p><p>In this thesis, we focus on three types of the boundary quantum phases : Kondo effects, boundary quantum phase transitions, and Majorana fermions. Our motivation is to design and prob those effects by using a important type of nanostructures, i.e. quantum dots. A vast variety of models related to quantum dots (QDs) are studied theoretically, which includes a QD coupled to a mesoscopic bath, a quadruple QD system with metallic leads, a QD with dissipative environments, and a QD coupled to a Majorana fermion zero mode.</p><p>Quantum dots provide a way to study the interplay of Kondo effects and mesoscopic fuctuations. In chapter 5, we consider a model including an Anderson impurity (small QD) coupled to a mesoscopic bath (large QD). Both the weak and strong coupling Anderson impurity problems are characterized by Fermi-liquid theories with weakly interacting quasiparticles. We find that the fluctuations of single particle properties in the two limits are highly correlated and universal : The distributions of the spectrum within the Kondo temperature collapse to universal forms; and the strong coupling impurity changes the wave functions corresponding to the spectrum within the Kondo temperature. </p><p>Quantum dots also bring the possibility to study more complex quantum impurities (multi-QDs) and the competition among dierent interactions, which may induce exotic effects: boundary quantum phase transitions and novel Kondo effects. In chapter 7, we design a quadruple quantum dot system to study the competition among three types of interactions: Kondo, Heisenberg, and Ising. We find a rich phase diagram containing two sharp features : a Berezinsky-Kosterlitz-Thouless type quantum phase transition between a charge-ordered phase and a charge liquid phase and a U(1)XU(1) Kondo state with emergent symmetry from Z2 to U(1). In chapter 8, we study a dissipative resonant level model in which the coupling of a fermionc bath competes with a dissipation-induced bosonic bath. we establish an exact mapping from this dissipative resonant level model to a model of a quantum dot embedded into a Luttinger liquid wire, and we also find two kinds of boundary quantum phase transitions (a Berezinsky-Kosterlitz-Thouless type and a second order type).</p><p>Finally, in chapter 9, we propose an experimental system to detect Majorana fermion zero modes. This system consists of a spinless quantum do coupled to a Majorana fermion which exists in the end of a p-wave superconductor wire. The Majorana Fermion strongly infuence the transport properties of the quantum dot. The zero temperature conductance peak value (when the dot is on resonance and symmetrically coupled to the leads) is e^2/2h. In contrast, if the wire is in its topological trivial phase, the result is e^2/h; if the side-coupled mode is a regular fermionic zero mode, the result is zero. Driving the wire through the topological phase transition causes a sharp jump in the conductance by a factor of 1/2. This result can be used to detect the existence of Majorana fermions.</p> / Dissertation

This thesis focuses on development of novel nanomaterials, including semiconductor quantum dots (QDs) and gold nanoparticles (AuNPs), for fabricating white-light emitting devices and assaying thiols in biological and environmental samples. The thesis mainly contains two divisions. One demonstrates synthesis, optical properties and white-light emissions of alloyed quantum dots and their application to light-emitting devices. The other describes to combine functionalized gold nanoparticles with capillary electrophoresis and accomplish high selectivity and ultrasensitive detection for thiols. First, through one-step aqueous synthesis, alloyed ZnxCd1¡VxSe QDs have been successfully prepared at low temperatures by reacting a mixture of Cd(ClO4)2 and Zn(ClO4)2 with NaHSe using 3-mercaptopropionic acid as a surface-stabilizing agent. The optical properties and composition of the alloyed QDs were highly dependent on the molar ratio of Zn2+ to Cd2+. With the increase in Zn content, a systematic blue shift occurred in the first exciton absorption and band edge emission. Moreover, X-ray diffraction peaks of the alloyed QDs systematically shifted to larger angles simultaneously. These systematic shifts indicated the formation of the alloyed QDs. Interestingly, among these alloyed QDs, Zn0.93Cd0.07Se QDs exhibited white-light emission with quantum yields of 12%. In addition, we discovered that we could adjust the relative strength of the band edge and trap state emissions by controlling the reaction time, thereby attain white-light-emitting QDs. Finally, we blended alloyed QDs with ultraviolet-transparent polydimethylsiloxane (PDMS) to develop a white-light, solid-state lighting device by using a 365-nm UV lamp as the pump source. In the other part of this thesis, we proposed a method for selective enrichment of thiols using Tween 20-capped gold nanoparticles (AuNPs) prior to capillary electrophoresis coupled with laser-induced fluorescence (CE-LIF). By forming Au-S bonds, Tween 20-AuNPs can selectively extract thiols from a complicated matrix. A Tween 20 capping layer not only suppresses nonspecific adsorption, but also enables NPs to disperse in a highly-salinity solution. For analyses of aminothiols, after extraction and centrifugation, thioglycollic acid was utilized to remove aminothiols that attached to the NP surfaces. The extracted aminothiols was derivatized with o-phthalaldehyde (OPA) followed by CE-LIF. The use of this nanoprobe provided approximately 11-, 282-, and 21-fold sensitivity improvements for homocysteine (HCys), glutathione (GSH), and £^-glutamylcysteine (GluCys), respectively. Furthermore, the limits of detection (LODs) at a signal-to-noise ratio of 3 for HCys, GSH, and GluCys are 4013, 80, and 383 pM, respectively. A practical analysis of aminothiols in human urine sample has been accomplished by our proposed method. For another application to determining thiol-containing peptides, we use dithiothreitol to remove thiol-containing peptides from the NP surface through ligand exchange. The released peptides are selectively derivatized with OPA to form tricyclic isoindole derivatives. After injecting a large sample volume, the sensitivity of these peptides was improved by stacking them via using polyethylene oxide (PEO) as additive for on-line concentration and separation. As a result, LODs for GSH, GluCys, and phytochelatins (PC2 ~ PC4) were down to 0.1-6 pM. The proposed method has the lowest LODs for five peptides compared to other reported methods, and it also detect dissolve thiols in seawater in practice. Our proposed method is capable of ultrasensitive detection for thiols in biological and environmental samples.

The purpose of the thesis is enhancing efficiency of asymmetric quantum dots (AMQD) solar cells. The AMQD structures are grown on the n-type GaAs substrate by (MBE). In order to enhance the photovoltaic characteristics, we introduce InGaAs quantum well (QW) and modulation doping in the well to investigate effect of the strain relief and built-in electric field in the active layer. In our experiment, we analyze the optical property of AMQD structures by photoluminescence measurement system, and then decompose emission wavelength by Gaussian fitting to find optical characteristics of each single layer quantum dots. Besides, we also measure photocurrent spectra, external quantum efficiency, electrical absorption, and electro reflectance spectra to discuss carrier transition inside AMQD structure . Finally, we acquire the photovoltaic basic parameter under one sun. The results show that QDs provide additional photocurrent via absorbing extra photons, but the open circuit voltage decrease seriously due to the accumulated strains. So as to relieve the strains and enhance carriers extraction, we introduce QW layers with different growth temperatures and change the modulation doping concentrations . From the results, the higher growth temperature for QW diminishes accumulated strains, and the higher p-type modulation doping concentration indicates an extraction enhancement due to the stronger built-in electric field. By optimizing QW growth conditions, the efficiency has overtaken GaAs baseline cells. In addition, we improve the photon-excited current collection by using matrix pattern and wet etching on the device surface, the best photovoltaic characteristic shows V OC = 0.74 V, J SC = 18.82 mA/cm2, FF = 0.78, £b= 10.86%.

New methods of biological analyte sensing are needed for development of miniature biosensors that are highly sensitive and require minimal sample preparation. One novel technique employs optical resonances known as Whispering Gallery Modes (WGMs). These modes arise from total internal reflection of light at the internal surface of a high index microsphere within a low index medium and produce an evanescent field that extends into the surrounding medium. The WGMs produce multiple narrow spectral peaks that shift position with variations in the local index of refraction sampled by the evanescent tail of the WGMs. To excite these WGMs, we embed quantum dots (QDs) in the periphery of polystyrene microspheres to serve as local light sources. By coupling emission from the QDs to the WGMs, the sensors can be excited and interrogated remotely and, by monitoring the shift of multiple resonance modes, may provide higher sensitivity and accuracy compared with similar techniques. The high refractometric sensitivity of the WGMs offers potential for trace detection of molecules adsorbed onto or bound to the microsphere sensor elements. The sensitivity of these sensors is demonstrated by monitoring the wavelength shift of multiple resonant modes as bulk index of refraction is changed. The potential for targeted biosensing is explored through addition of a protein that adsorbs to the microsphere surface, thrombin. Microsensor response in all cases demonstrated increased sensitivity over theoretical predictions. Models based on Mie theory and continuity of the radial functions across the sphere-media interface were used to model the location, Q-factor, and sensitivity of the WGMs in microspheres by considering the embedded QDs as a high index outer layer. This model was used, along with estimates of the QD-layer index and penetration depth, to relate the locations and sensitivities of the modes to our experimental results with strong agreement between the two. In all, these microspheres demonstrate feasibility for use as remote microsensors with sensitivities rivaling current techniques.

Large bone defects pose a significant clinical challenge currently lacking an adequate therapeutic solution. Bone tissue engineering (BTE) therapies aim to provide that solution by combining structural scaffolds, bioactive factors, and/or osteogenic cells. Cellular therapies are likely vital to repair severe defects in patients lacking sufficient endogenous cells. Stem cells are attractive cell choices due to their osteogenic differentiation and extensive proliferation abilities, but their therapeutic potential is still uncertain, as studies comparing stem cell sources and delivery methods have produced inconsistent results. In this thesis, we developed a challenging in vivo large bone defect model for quantitative comparison of human stem cell-based therapies and then evaluated the abilities of adult or fetal stem cell-seeded constructs to enhance defect repair, with or without added osteogenic cues. First, we showed that cellular construct treatment enhanced defect healing over acellular construct treatment, although there were no differences between adult or fetal cell sources. We next labeled stem cells with a fluorescent tracking agent, the quantum dot, to determine biodistribution of implanted cells during the repair process. While quantum dots effectively labeled cells in vitro, they were ineffective in vivo tracking agents due to false positive signals and detrimental effects on stem cell-mediated repair. Finally, we developed a novel gene therapy technique using virus-coated scaffolds to deliver the osteogenic factor bone morphogenetic protein 2 (BMP2) to defect sites, either by in vitro (BMP2 transduction of seeded stem cells pre-implantation) or in vivo (BMP2 transduction of defect-site host cells) means. While defect-site BMP2 delivery through gene therapy methods improved repair, in vivo therapy enhanced healing more than stem cell-based in vitro therapy. This finding does not rule out the potential of stem cell-based in vitro gene therapy treatment for functional bone repair, as increases in viral dose may improve stem cell-mediated healing, but it does present evidence of a novel acellular BTE therapy with potential off-the-shelf clinical application in large bone defect repair, as scaffolds could be virally coated with the gene for BMP2 expression and frozen until implantation.

The low-energy properties of quantum dot systems are dominated by the Kondo effect. We study the dependence of the characteristic energy scale of the effect, the Kondo temperature, on the gate voltage, which controls the number of electrons in the strongly blockaded dot. We show that in order to obtain the correct Kondo temperature as a function of the gate voltage, it is crucial to take into account the presence of many energy levels in the dot. The dependence turns out to be very different from that in the conventional single-level Anderson impurity model. Unlike in the latter, the Kondo temperature cannot be characterized by a single parameter, such as the ratio of the tunneling-induced width of the energy levels in the dot and the charging energy.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed March 12, 2010). Includes bibliographical references. Also issued in print.

This Ph.D work reports the studies of photovoltaic devices produced by solution processable methods. Two material systems are of interest: one is based on organic semiconductors, and another on organic/inorganic hybrid composites. Specifically, organic photovoltaic (OPV) devices are made using photoactive materials consisted of a -conjugated polymer [Poly(3-hexylthiophene), or P3HT] and fullerene derivative [phenyl-C60-butric acid methyl ester, or PCBM] in a bulk heterojunction (BHJ) structure of donor/acceptor network. On the other hand, hybrid photovoltaic (HPV) devices are made from blend of quantum dots and -conjugated polymers. The QD material presented here are of the lead sulfide (PbS), and lead selenide (PbSe), whereas the polymers are either P3HT or Poly(3-dodecyl thienylene vinylene) (PTV)with controlled regio-regularity. For OPV devices, two different device geometries are investigated, namely, the conventional or normal structure where indium tin oxide (ITO) is used an anode, and a metal cathode is fabricated by thermal vapor deposition (TVD). In this geometry, thin layer (about 30~35nm) of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is deposited from aqueous solution onto ITO as hole transport layer (HTL). The second geometry, called the inverted structure, uses ITO as the cathode of the device. A thin layer of cesium carbonate (Cs2CO3) (about 1~2nm) is applied over the ITO and functions as electron transport layer (ETL), thereby decreasing the work function of ITO and allowing it to function as the cathode. In this case, PEDOT:PSS is mixed with 5vol.% of dimethylsulfoxide to increase conductivity for serving as anode. Two solution processable methods, spin-coating and spray processes were investigated, and a detailed study of nanomorphology influence under different annealing conditions, different solvents and thickness are reported. The main contribution of this Ph.D. work was the development and implementation of a layer-by-layer (LBL) all-spray solution-processable technique to fabricate large-scale OPV arrays, with more than 30% transmission in the visible to near IR range. Comparing with traditional laboratory OPV fabrication based on spin-coating and using metal as cathode contact, which greatly limits transparency of solar cells and posts difficulty for large scale manufacturing, this LBL spray process solves these two problems simultaneously. This technology eliminates the need for high-vacuum, high temperature, low rate and high-cost manufacturing associated with current silicon and in-organic thin film photovoltaic products. Furthermore, this technology could be used on any type of substrate including cloth and plastic. Single cell OPV with active area of 4mm2 was used as preliminary test device to obtain fabrication parameters for multi-cell OPV arrays. Three different sizes of OPV arrays were fabricated and tested under various illumination conditions. Starting from a 4" x 4" array with 50 cells in series connection 4" x 4" substrate consisting of 50 cells with total active area of 30cm2, a scaled up 1' x 1' array was fabricated as a proof of concept, and whose results are reported. Scaled down arrays, called micro arrays, are also presented in this work. OPV micro array has the potential application in DC power supplies for electrostatic Microelectromechanical systems (MEMS) devices. The first generation micro array consists of 20 small (1mm2) solar cells connected in series for a total device area of approximately 2.2cm2. The 2nd generation micro array with 60 cells shares the same size substrates and single cell active area as the first generation. However, the 2nd generation micro array cell has a new design with reduced series resistance and improved cell occupancy by 3 fold. Infrared quantum dots (QD) such as PbS and PbSe have potential in photovoltaic applications. These solution processable quantum dots with tunable electronic properties offer very attractive approach for expanding spectral sensitivity of -conjugated polymers to infrared region of solar spectrum. However, these QDs often have defects originated from either incomplete surface passivation or imperfections in the quantum Dots. The electronic levels of defects often are within the bandgap of the semiconductor. These in-gap states are of great importance since they affect the final destiny of excitons. Continuous wave photoinduced absorption spectroscopy has proven to be a convenient and successful technique to study long-lived photoexcitations of in-gap states. Part of this Ph.D work was the investigation of a peculiar gap state found in films of PbS QDs. This gap state bears confinement dependence, with a lifetime about 2μs. A detailed analysis of the Stokes shift, temperature dependence of PL, absorption and photoinduced absorption reveals the unconventional GS is a new state of a trapped exciton in a QD film. This gap state is directly relevant to exciton dissociation and carrier extraction in this class of semiconductor quantum dots. As synthesized PbSe and PbS quantum dots usually have bulky ligands such as oleic acids or TOPO (trioctylphosphine oxide). This capping layer is necessary to prevent nanocrystals from coalescence, however, the bulky ligands hinder charge extraction from and charge transport through the nanocrystals, as well as exciton dissociation at the nanocrystal/polymer interface. Common ways to manipulate ligands include ligand wash and ligand exchange in solution, and ligand removal on films. Through this Ph.D. work, a novel method using electric field to manipulate quantum dots ligands for interface of quantum dots and polymer, which possibly could facilitate charge extraction from the quantum dots and charge transfer between quantum dots and polymers, without the need of harmful chemicals. Over four orders improvement of photoconductivity at zero bias and more than six orders improvement at 5V reverse bias in a sandwich structure quantum dots photovoltaic device, and more than 5x improve in film smoothness. After thorough fundamental study on QD optoelectronic properties, hybrid photovoltaic (HPV) device was fabricated using a blend solution of PbS QDs and P3HT. Two different solution processes are used to form the QD/polymer active layer, one is the traditional spin coating method, and another is the spray technique developed in this Ph.D. Work. Different film morphology was observed with these two methods. Although the film is slightly rougher in the case with sprayed QD/polymer active layer, the phase segregation is more distinct and with smaller domain, which is beneficial for charge transport.

Obtaining abundant, clean, sustainable energy has become an increasingly large need globally. To date, solar cells have had a limited impact in meeting energy demands. This is primarily due to their relatively high cost and low power conversion efficiencies. Sensitized solar cells, or Grätzel cells, have the potential for being made with low cost materials, and achieving power conversion efficiency high enough to economically compete with fossil fuels. Understanding the dynamics of charge carriers as they separate at the interface of the light absorbing donor and their semiconducting acceptor becomes an important first step in the realization of an inexpensive and efficient sensitized solar cell. Presented is the theory of treating electrons at donor-acceptor interfaces, and why time-resolved surface second harmonic generation (TR-SHG) is used to probe the dynamics of charge carriers at these interfaces. A series of experiments are described where various preparations of thin films of sensitizers on single crystal titanium dioxide, a common acceptor in Grätzel cells, are prepared and studied. TR-SHG studies of thin films of colloidal PbSe and CdSe QDs showed remarkably different electron cooling and transfer dynamics. The electron cooling in PbSe is thermally activated in PbSe QDs. By cooling samples, electron transfer from higher excited “hot” states was observed. Contrary, for CdSe QDs electron transfer rates were dependent on the energy of the excited state. When higher states were excited, charge transfer rates decreased, indicating that only low energy, electrically “cold”, states participate in charge transfer. When carbon based grapheme QDs are used, the electron dynamics mimic PbSe QDs. In this system, increasing the pump energy leads to slower recombination rates, indicating that electrons have to drift further back to the interface. / text

Advances in nanofabrication have bolstered the development of new optical devices with potential uses ranging from conventional optoelectronics, such as lasers and solar cells, to novel devices, like single photon or entangled photon sources. Quantum encryption of optical communications, in particular, requires devices that couple efficiently to an optical fiber and emit, on demand, indistinguishable photons. With these goals in mind, ultrafast spectroscopy is used to study the electron dynamics in epitaxially grown InAs/GaAs quantum dots (QDs). Quantifying the behavior of these systems is critical to the development of more efficient devices. Studies of two newly developed InGaAs QD structures, quantum dot clusters (QDCs) and QDs embedded in photonic wires, are presented herein. GaAs photonic wires with diameters in the range of 200 to 250 nm support only the fundamental HE11 guided mode. To fully quantify these new systems, the emission dynamics of QDs contained within wires in a large range of diameters are studied. Time correlated single photon counting measurements of the ground state exciton lifetimes are in very good agreement with predicted theoretical values for the spontaneous emission rates. For diameters smaller than 200 nm, QD emission into the HE11 mode is strongly inhibited and non-radiative processes dominate the decay rate. The best small diameter wires exhibit inhibition factors as high as 16, on par with the current state of the art for photonic crystals. The QDCs are the product of a hybrid growth technique that combines droplet heteroepitaxy with standard Stranski-Krastanov growth to create many different geometries of QDs. The work presented in this dissertation concentrates specifically on hexa-QDCs consisting of six InAs QDs around a GaAs nanomound. The first ever spectral and temporal properties of QDs within individual hexa-QDCs are presented. The QDs exhibit narrow exciton resonances with good temperature stability, indicating that excitons are well confined within individual QDs. A distinct biexponential decay is observed even at the single QD level. This behavior suggests that non-radiative decay mechanisms and exciton occupation of dark states play a significant role in the recombination dynamics in the QDCs. / text